ci/cache project root cloudbuild.yaml fix (#3566)

When setting up a trigger for this cloudbuild.yaml file, it can only run
from the repository root - which then means this script fails as files
aren't where they are expected.

This fix retains the original structure of the cloudbuild.yaml and
shifts the original source files into where the rest of the script
expects it.

build/build-image/cache/cloudbuild.yaml

@@ -15,6 +15,18 @@
 
 steps:
 
+    #
+    # Move everything to the /workspace as this is what the canonical
+    # build script uses, and we want to be as close to that as possible.
+    #
+  - name: bash
+    id: setup-cache-files
+    script: |
+      mkdir /tmp/workspace && \
+      mv /workspace/* /tmp/workspace && \
+      mv /tmp/workspace/build/build-image/cache/* . && \
+      ls -l
+
   - name: 'gcr.io/cloud-builders/docker'
     id: build_base_image
     args: